Graphics quality difference between MacBook Pro 2015 and Mac mini when connected to external monitor

Hi there, I hope someone can help me with my query.

I've connected a MacBook Pro 2015 (via HDMI) and a Mac mini 2018 (via Displayport) to a Samsung LC34J791 using Picture-by-Picture mode. For some reason the MacBook seems to output a slightly sharper image, almost like it has deeper blacks or something. Both Macs have the same monitor calibration settings and the Monitor has the same settings for each source. The MacBook Pro is on Big Sur and the Mini is on Catalina. I have tried using the HDMI cable with the Mac Mini and doesn't improve the image.

I have included a side by side image of the same webpage using both sources. Perhaps the difference is negligible to some, and in honesty it doesn't show quite as well on the images I've included, but I think it's still visible.

I am aware that the Mini's graphics chip s not the greatest and I'm wondering if that's what's making the difference? I have heard that investing in an eGPU for the Mac mini is a really good option to boost graphic performance, however I am wondering if 'performance' is simply speed, or if it improves the overall graphic/colour output as well? 

I can totally get by with the Ma mini's image quality, but seeing the MacBook Pro image sharpness makes me want to be able to have that quality with the mini too.

Many thanks in advance

Sam